A wide variety of coatings have been used to coat the surfaces of food and beverage containers. For example, metal cans are sometimes coated using coil coating or sheet coating operations, that is, a coil or sheet of a metal substrate, for example, steel or aluminum, is coated with a suitable composition and cured. The coated substrate is then formed into the can body. Coatings for food and beverage containers must be capable of high speed application to the substrate while providing the necessary properties when cured to perform in a demanding end use. For example, the coating should be safe for food contact, be sufficiently flexible to withstand the forming operation and have excellent adhesion to the substrate.
Many of the coating compositions for food and beverage containers are based on epoxy resins that are the polyglycidyl ethers of bisphenol A. Bisphenol A in packaging coatings either as bisphenol A itself (BPA), derivatives thereof, such as diglycidyl ethers of bisphenol A (BADGE), are problematic. Although the balance of scientific evidence available to date indicates that small trace amounts of BPA or BADGE that might be released from existing coatings does not pose health risks to humans, these compounds are nevertheless perceived by some as being harmful to human health. Consequently, there is a strong desire to eliminate these compounds from coatings for food and beverage containers.
Compositions comprising polyesters and resole resins have been investigated as substitutes for the bisphenol A-containing epoxy resins. Although these compositions have good cured coating properties, such as solvent resistance and corrosion resistance and inertness to many foods and beverages, they have insufficient flexibility to withstand the forming process leading to cracks and fissures in the cured coating resulting in coating failure.
The present invention overcomes these problems by providing a polyester-resole composition that has enhanced flexibility while maintaining good cured coating properties.